The vertical cross was known in ancient times as a good luck symbol. In German it was named the Haken Kreuz. In WWII Germany is was turned at a 45 degree angle and the arms turned to the right it was named the swastika. The interpretation of the symbols goes back to Voss' days as a Hussar. Their motto was "How do we fight? With heart, luck and iron will". Hence the heart, Haken Kreuz with the iron cross. It is read from the left side because that is how you mounted a horse in the cavalry. Note the foot step is also on the left side of the fuselage.
Here is a bit of fun on the subject.
Click here.